1. Field of the Invention
This invention relates to compositions for use in well-working operations such as drilling, workover and completion, packing and the like, well-working processes utilizing such compositions, and an additive to prevent, inhibit or alleviate differential or mechanical sticking of downhole equipment in well boreholes.
In the drilling of oil wells, gas wells, injection wells and other boreholes, various strata are bypassed in achieving the desired depth. Each of these subsurface strata has associated with it physical parameters, e.g., porosity, liquid content, hardness, pressure, etc., which make the drilling art an ongoing challenge. Drilling through a stratum produces an amount of rubble and frictional heat; each of which must be removed if efficient drilling is to be maintained. In rotary drilling operations, a string of drill pipe having a drill bit mounted on the lower end thereof is rotated to cause the bit to make the hole. Heat and rock chips are removed by the use of a liquid known as drilling fluid or mud. Typically, drilling fluid is circulated down through the drill string, out through orifices in the drill bit where it picks up rock chips and heat and returns up the annular space between the drill string and the borehole wall to the surface. There it is sieved, reconstituted and directed back down into the drill string. The rotation of the drill string and circulation of the drilling fluid are substantially continuous while drilling, being interrupted for essential operations such as adding an additional section of drill pipe to the top of the drill string or when the entire string is disassembled and pulled from the well bore (called "tripping"). Periodically during interruptions in the drilling operation and also at its conclusion, downhole tools such as logging tools are inserted into the bore and subsequently recovered, and casing is inserted into the bore and set.
The flow properties of the drilling fluid play a vital role in the success of the drilling operation. These properties are primarily responsible for the removal of drill cuttings but influence drilling progress in many other ways. Unsatisfactory performance can lead to such serious problems as bridging the hole, filling the bottom of the hole with drill cuttings, reduced penetration rates, hole enlargement, stuck pipe, loss of circulation, and even a blowout.
Drilling fluid may be as simple in composition as clear water or it may be a complicated mixture of clays, thickeners, dissolved inorganic components, and weighting agents. The characteristics of the drilled geologic strata and, to some extent, the drilling apparatus determine the physical parameters of the drilling fluid. For instance, while drilling through a high pressure layer, e.g., a gas formation, the density of the drilling fluid must be increased to the point that the hydraulic or hydrostatic head of the fluid is greater than the downhole pressure of the stratum to prevent gas leakage into the annular space surrounding the drill pipe and lower chances for a blowout.
The particle size in common drilling fluids is, as general rule, from about 0.5 to 5 microns, with a small percentage (perhaps as much as 5%) of the particles being as large as 325 mesh (44 microns). The balance of the particles above this range are removed in process of preparation and in separation of the rock cuttings prior to recirculation of the drilling fluid. Because of the constant cleaning and removal of larger particles, the drilling fluid can bridge only very small fissures (less than 0.002 in.) within the formations as the muds are normally used.
In strata which are porous in nature with openings or fissures larger than about 0.001 to 0.002 inch and additionally have a low formation pressure, another problem occurs. Some of the drilling fluid, because of its hydrostatic head being greater than the formation pressure, migrates out into the porous layer rather than completing its circuit to the surface. One common solution of this problem is to use a drilling fluid which contains bentonite clay or other filtration control additives. Typically, such additives have particles of a size slightly smaller than the size of the pore openings of the formation. The porous formation tends to filter the filtration control additive from the drilling fluid and forms a filter cake on the borehole wall thereby preventing the outflow of drilling fluid. The liquid which enters the formation while the cake is being established, is known as the surge loss or spurt loss, while the liquid which enters after the cake is formed is known as the drilling fluid filtrate. The permeability of the filter cake is directly related to the particle size distribution in the drilling fluid, and in general, the cake permeability decreases as the concentration of particles in the colloidal size range increases. See House, U.S. Pat. No. 5,004,553. As long as this filter cake is intact, very little fluid is lost to the formation.
In general, it is most economical to use a water-based fluid over an oil-based drilling fluid, from the standpoint of original cost, maintenance and protecting the ocean environment. Accordingly, oil-based drilling fluids are not desirable, unless essential to the drilling operation. Many formation evaluation, or well logging, tools depend upon the use of water-based drilling fluids because such fluids are electrically conductive through the earth formation, rather than insulative, as in the case of oil-based drilling fluids.
It is known by those skilled in the art that stuck pipe is one of the most common hazards encountered in drilling operations. One of the primary causes of drill string "sticking" is the effect of differential pressure between the hydrostatic head of the drilling fluid column in the well bore and any porous, low-pressure earth formations through which the drill string passes. Under such conditions, the pressure difference presses the drill pipe against the borehole wall with sufficient force to prevent movement of the pipe. This occurs because the density or weight of the drilling fluid in the well bore creates a hydrostatic pressure against the pipe that is substantially greater than that in a porous earth formation traversed by the well bore. This is due to the filtrate (typically water in the drilling fluid) flowing through the desirable "mud cake" and the well bore wall into the low pressure formation. This condition may occur in the drill collar section of the drill string which is used to apply weight to the bit directly above the drill bit and when the return drilling fluid flow around the smaller diameter drill string is less turbulent and hence relatively laminar. Thus, where the drill pipe lies close to one side of the well bore, as in slant holes, higher differential pressure across the drill pipe increases its adherence to the side of the well bore. In a worst case, this results in differential pressure sticking of the drill string. However, differential sticking of pipe more frequently occurs after circulation and rotation have been temporarily suspended, as when making a connection.
Correction of drill string sticking conditions usually requires a decrease in the drilling fluid pressure in the well either by reducing the hydrostatic head of the drilling fluid or increasing solids content of the fluid to reduce filtrate loss, with subsequent building of a thicker filtercake to increase the pipe contact area. Alternatively, sticking can sometimes be avoided by using smaller diameter drill pipe, or fewer drill collars in the weight assembly above the bit. The problem of differential pipe sticking is frequently severe where a well encounters over-pressured formations. In such wells, the formation pressure exceeds the pressure to be normally expected due to hydrostatic head alone at that depth. In such wells passing through overpressured formations the counterbalancing hydrostatic pressure in the well cannot be reduced safely at deeper depths when an under-pressured formation is encountered. However, such greater pressures on deeper formations may substantially increase the risk of fracturing the formation, with accompanying loss of drilling fluid from the well into the fracture, and creating potential well blow-out.
It is also known that frequently a drill string may stick in a drilling well because of mechanical problems between the drill string and the well bore itself. Such a condition can sometimes occur in what is known as the "keyseat effect". That is, a keyseat is created when the drill string collar or a pipe joint erodes a circular slot the size of the drill pipe tube or tool joint outside diameter in one side of the larger circular bore hole, as originally cut by the drill bit. Such a slot can create greatly increased friction or drag between the drill string and the earth formation and result in seizure of the drill collars when an attempt is made to pull the string out of the hole and the collars become wedged in the keyseat. Such problems can also be created by excessive weight on the drill string so that the drill string buckles in the lower section and particularly where the bore hole has a high angle, say in excess of 60.degree. from vertical, or the well bore includes more than one change of direction, such as an S-curve or forms one or more "dog-legs" between the drilling platform and the drill bit. It is also known that in mechanical sticking of drill string, earth formations around the well may be sufficiently unstable so that the side wall collapses or sloughs into the well bore and thereby sticks the pipe.
Economics also play a significant role in drilling wells with potential sticking tendencies. C. C. Newhouse points out that analysis of stuck pipe data for Chevon's Gulf of Mexico operations during 1988 showed that each incident cost an average of $700,000.00. He further states that although the reservoir pressures are known, depleted sands can present serious drilling problems and substantial cost increases. Differential sticking of drill pipe, casing and logging tools; and tight hole conditions due to excessive filtrate loss and cake buildup are some of the problems which may occur. Additional casing strings may also be required to cover these zones. Effectively plugging the depleted intervals is the key to preventing most of these problems. See C. C Newhouse, "Successfully Drilling Severely Depleted Sands," Paper SPE 21913 Presented at the 1991 SPE/IADC Conference held in AmsterDam, 11-14 Mar. 1991.
2. Prior Art
The extent of each pipe sticking problem generally depends upon the amount of time the operator is willing to "wash over" the stuck section of the drill pipe (after unthreading and removal of the unstuck portion), or to "fish" by otherwise manipulating the drill string. Correction may also include spotting or completely replacing the water-based drill fluid with a spotting agent. The spotting agents are often oleophilic compositions and may be oil-based drilling fluids, invert emulsions of water in oil, or a material as readily available as diesel oil. After the slug of, typically, 10-15 barrels of spotting agent is introduced, addition of drilling fluid is re-commenced. The slug of spotting agent continues its trip down through the drill string, out the drill bit, and up the wellbore annulus until it reaches the site of the stickage. Upon arrival of the spotting agent at the stickage location, circulation is temporarily ceased. Those skillful in this art speculate that oil-based spotting agents tend either to dehydrate the filter cake on the borehole wall and cause it to break up or to reduce the coefficient of friction between the filter cake and the drill string, thereby allowing the drill string to come free. In any event, once movement of the drill string is detected, circulation of the drilling fluid is restored. It should be observed that the cost of this process is high and the success rate only moderate. Furthermore, this method does not inhibit the occurrence or reoccurrence of differential sticking. Failure to free the drill string results either in abandoning the well bore or side tracking the bore hole above the stuck point. This may include loss of the drill bit, collars and stuck lengths of pipe in the bore hole.
The problem of sticking pipe has been described in numerous publications in the literature, particularly as it relates to differential sticking of the drill string to the well bore, that is, adherence of the drill string against a porous formation so that there is no circulation of drilling fluid around one side of the drill string. Such literature is in part directed to the use of various pieces of drilling equipment or drilling procedures, which typically involve a higher capital expense. The following are examples of such drilling equipment and procedures.
Dellinger, U.S. Pat. No. 4,428,441, proposes the use of noncircular or square tool joints or drill collars, particularly in the drill string directly above the drill bit. Such shape assures that circulation is maintained around the drill pipe and reduces the sealing area between the pipe and the side wall where the differential pressure may act. However, such tools are expensive and not commonly available. Further, they may tend to aggravate the keyseat problem in relatively soft formations since the square edges of such collars may tend to cut the side wall in high angle holes.
Lawrence, U.S. Pat. No. 4,298,078, proposes using a special drill section directly above the drill bit to permit jarring the drill bit if the pipe tends to stick. Additionally, valves in the tool may be actuated to release drilling fluid around the drill string to assist in preventing or relieving a stuck drill string condition.
Steiger, U.S. Pat. No. 4,427,080, is directed to binding a porous layer on the outside of the drill string. Such a coating is stated to prevent differential pressure sticking of the pipe by increasing liquid flow around the drill string.
Messenger, U.S. Pat. No. 4,368,787, proposes reversing the circulation of the drilling fluid using a pump powered by the cones of the rotary bit. The drilling cuttings are thus removed by pumping the drilling fluid up and out the drill pipe and therefor kept out of the annulus.
Cloud, Canadian Patent 1,253,846, proposes mitigating the problems of differential pressure sticking and of packoff in the borehole upon removal of the drill string by using upwardly or backwardly directed jets, for example, in the drill collars or heavy weight pipe joints to aid in the removal of cuttings from the borehole.
Alternatively, such literature discloses the use of methods to avoid differential sticking by assuring that the drilling fluid is tailored to match the earth formations penetrated by the well bore. However, in drilling new or deep wells, where intimate knowledge of the formations is not available, and particularly where low pressure formations are to be encountered, it is difficult to predict and take corrective, or preventive, action prior to such drill pipe sticking. Further, while these problems can be avoided by deeper casing of the bore hole around the drill string, such casing is expensive and in general undesirable, because it limits formation evaluation with conventional well logging tools. As noted above, this also is a primary reason that oil-based drilling fluids (which are insulative) are not desirable, unless essential to the drilling operation.
Examples of drilling fluids additives used to avoid or remedy stuck pipe include the following:
Moses, U.S. Pat. No. 4,423,791, discloses avoiding differential sticking by use of glass beads in the drilling fluid to inhibit formation of a seal by the filter cake between the drill string and the well bore adjacent a low pressure zone. The glass beads have a particle size of 9.84 to 187 mils (U.S.A. Sieve Series 4 to 60).
Two other types of specialty products which have been used in preventing differential sticking while drilling depleted sands are Gilsonites (Registered trade mark of the American Gilsonite Company) and cellulose fibers. In severe conditions, one or both products may be maintained in the mud system. Although both products improve the plugging efficiency of drilling fluids, they function in a different manner.
Gilsonite is a naturally occurring carbonaceous material that is classified as an asphaltite and has a softening point of 330.degree.-400.degree. F. Its primary use is to improve borehole stabilization by sealing off microfractures and forming a plating film on the borehole face. The malleable and insoluble nature and PSD (particle size distribution) of Gilsonite are the means by which it is effective in sealing permeable formations and improving the filter cake quality. Field use has shown that it helps reduce HTHP fluid loss and can reduce torque and drag.
This product is normally maintained in concentrations up to 4 lbm/bbl and is most effective if added before penetrating the troublesome interval. The kickoff point normally provides a good place to start additions of Gilsonite, since it can aid in lubricity. This product also functions well with cellulose fiber products, but should be pilot tested to ensure the optimum concentrations.
Cellulose fibers have also been used in plugging permeable formations. Most were formulated to control seepage loss and differential sticking instead of shale stabilization like the Gilsonite. They typically function by swelling and wedging into the pore throat once exposed to a differential pressure. This has proven beneficial in decreasing mechanical erosion. Another advantage of this product is that it is very effective if used in sweeps, even after problems begin. Hydration of the product in water and caustic before adding it to the system is beneficial in high solids muds. Since several manufacturers offer their fiber products in coarse and fine grinds, the PSD of the drilling fluid can be adjusted to accommodate a wide range of permeabilities.
Concentrations when maintaining cellulose fiber products in the system, range between 3-8 lbm/bbl (PPB). Fiber products can be added at any point in the hole since they are not temperature dependent. Additions should begin early enough to ensure the optimum concentration is being maintained before drilling the depleted interval. They can be added much like the Gilsonite products, sweeps first. These products are compatible with virtually all water-based and oil-based mud systems. A few precautions to note about these products is that they will decrease alkalinities and can ferment if the pH is below 9.
In less severe cases, sweeps formulated in the drilling fluid are frequently used instead of maintaining the product in the system. They normally contain between 10 and 25 lbm/bbl. This method has proved effective in plugging off small troublesome areas and massive sands when the mud weight is low. They are also effective if pumped just after drilling through the depleted zone, while circulating the hole clean before a trip, or before pulling out of the hole to log. This will ensure a complete PSD is available to decrease the filter cake permeability. The rheology and alkalinities are checked prior to pumping the sweep. Additions of water and/or thinners may be required to lower rheological properties.
In extreme cases, spotting pills in the open hole before a trip can improve conditions once drilling is resumed. They contain lower concentrations than sweeps to prevent dehydration of the mud.
Another important factor is that the smallest shaker screens desired cannot always be used since they can strip the larger products from the system. Screen sizing should be based on the PSD of the product and the cut point of the screens. As a result, more dilution is typically required to control the drilled solids. Actual screen sizing should be determined by evaluating the cuttings on the shakers.
There are cheaper products; such as calcium carbonate, nut plug, and mica, that can be used to improve the PSD of the drilling fluid. They may not necessarily improve the plugging performance of the fluid because they are not deformable and do not swell upon entering the pore throats. These products can actually get in the way of the beneficial solids and act as a proppant.
As noted above, most cellulose fibers were formulated to control seepage loss (or loss circulation) and differential sticking. In order to combat or prevent lost circulation, it has been common in the past to add any number of materials to the drilling fluid which act to reduce or prevent flow of the drilling fluid outwardly into a porous stratum by reducing or limiting the permeability of the formation being drilled, thereby arresting a lost circulation condition. Such prior known cellulose fiber materials include fibrous, flake and granular ground forms thereof. Representative of such cellulose fibers include nut and seed shells or hulls (pecan, almond, walnut, peach, brazil, coconut, peanut, sunflower, flax, cocoa bean, cottonseed, rice, linseed, oat). See House et al., U.S. Pat. No. 5,004,553. See also Borchardt, U.S. Pat. No. 2,799,647 (almond shells); Gockel, U.S. Pat. Nos. 4,460,052 and 4,498,995 (granular: walnut shells, pecan shells, almond shells; fibers: sunflower seed-hulls, cottonseed hulls); Kern et al., U.S. Pat. No. 3,228,469 (coarse almond shells); Walker, U.S. Pat. Nos. 4,614,599, 4,635,726 and 4,664,816 (almond hulls, walnut hulls); Surles, U.S. Pat. No. 4,964,465 (almond hulls, walnut hulls); Delhommer et al., U.S. Pat. Nos. 4,633,950 and 4,704,213 (almond hulls, walnut hulls). Delhommer, Surles and Walter also cite Gockel '995. Of the foregoing cellulose fiber materials, peanut shells and walnut shells are known to have been used for differential sticking problems with moderate success. The other cellulose fiber materials have been tried, but apparently without much success.